On dynamic and thermodynamic components of cloud changes

Abstract

Clouds are sensitive to changes in both the large-scale circulation and the thermodynamic structure of the atmosphere. In the tropics, temperature changes that occur on seasonal to decadal time scales are often associated with circulation changes. Therefore, it is difficult to determine the part of cloud variations that results from a change in the dynamics from the part that may result from the temperature change itself. This study proposes a simple framework to unravel the dynamic and non-dynamic (referred to as thermodynamic) components of the cloud response to climate variations. It is used to analyze the contrasted response, to a prescribed ocean warming, of the tropically-averaged cloud radiative forcing (CRF) simulated by the ECMWF, LMD and UKMO climate models. In each model, the dynamic component largely dominates the CRF response at the regional scale, but this is the thermodynamic component that explains most of the average CRF response to the imposed perturbation. It is shown that this component strongly depends on the behaviour of the low-level clouds that occur in regions of moderate subsidence (e.g. in the trade wind regions). These clouds exhibit a moderate sensitivity to temperature changes, but this is mostly their huge statistical weight that explains their large influence on the tropical radiation budget. Several propositions are made for assessing the sensitivity of clouds to changes in temperature and in large-scale motions using satellite observations and meteorological analyses on the one hand, and mesoscale models on the other hand.

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Notes

  1. 1.

    Although we use, in practice, finite intervals of ω of 10 hPa/day to define dynamical regimes, we will consider in the following notations that ω intervals are infinitesimal

  2. 2.

    The discretized form of this equation, that is used in practice here, is of the form: \( \overline{\delta C} = \sum\nolimits_\omega {C_\omega \Delta P_\omega} + \sum\nolimits_\omega {P_\omega \Delta C_\omega} + \sum\nolimits_\omega {\Delta P_\omega \Delta C_\omega } \)where ΔC ω and ΔP ω refer to the changes in C ω and P ω

  3. 3.

    The change in cloudiness that occurs in a particular region of the tropics may result from both local and remote influences. At first approximation, if one considers that the remote effects are felt by clouds mostly through changes in the large-scale atmospheric motion, then the changes in cloud properties that occur for a given dynamical regime (the thermodynamic component) can be considered as being much less dependent on remote effects. We insist however that this is only an approximation: remote effects may affect clouds through other factors than a change in ω (a change in the temperature lapse rate, the occurrence of dry intrusions in the mid troposphere, etc)

  4. 4.

    Note that the change in ω that occurs in a given region results from both local and remote influences

  5. 5.

    For the current climate, Sud et al. (1999) suggested these temperature thresholds to be constrained by the relationship between the vertical profiles of dry and moist static energy in the tropical atmosphere. Figure 8 suggests that the SST thresholds derived from this relationship might depend on the mean tropical temperature

  6. 6.

    The reason for the decrease of convective activity or large-scale convergence at very high temperatures, pointed out and characterized by Waliser and Graham (1993) and Waliser (1996), is still a matter of study: it has been proposed that it could be related to the subsidence induced by remote forcings such as the large-scale subsidence associated by intraseasonal waves over the warm pool (Bony et al. 1997; Lau et al. 1997), or by a positive feedback between tropical convection and water vapour (Tompkins 2001)

  7. 7.

    Large-scale motions are sensitive to horizontal gradients in the boundary-layer entropy (Lindzen and Nigam 1987, Emanuel et al. 1994). The relationship between SST and boundary-layer entropy, and its link to the sharp increase of deep convection for SSTs above 26 °C are discussed by Sud et al. (1999) and Folkins and Braun (2003)

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Acknowledgements.

This work benefited from discussions with Kerry Emanuel, Jean-Yves Grandpeix, Christian Jakob, Laurent Li, Mark Webb and Yun-Ichi Yano, and from comments by anonymous reviewers. Part of this study was supported by the Environmental Program of the Commission of the European Communities (project ENV4-CT95-0126 entitled Cloud Feedbacks and Validation). French participants acknowledge the Programme National d’Etude Du Climat (PNEDC).

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Correspondence to S. Bony.

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Bony, S., Dufresne, JL., Le Treut, H. et al. On dynamic and thermodynamic components of cloud changes. Climate Dynamics 22, 71–86 (2004). https://doi.org/10.1007/s00382-003-0369-6

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Keywords

  • Outgoing Longwave Radiation
  • Cloud Property
  • Cloud Radiative Force
  • Cloud Response
  • Climate Perturbation